Micro-electromechanical device having a laminated thermal bend actuator

ABSTRACT

A micro-electromechanical device includes a substrate containing drive circuitry. An elongate actuator is fast with the substrate at a fixed end. The elongate actuator has a laminated structure of at least one inner layer and a pair of opposed, outer layers. The outer layers have substantially the same thermal expansion and elasticity characteristics. One of the outer layers defines an electrical heating circuit that is in electrical contact with the drive circuitry to be heated and to expand on receipt of an electrical signal from the drive circuitry and to cool and contract on termination of the signal, thereby to generate reciprocal movement of the actuator.

CROSS REFERENCES AND RELATED APPLICATIONS

The present application is a continuation of U.S. application Ser. No.10/401,987 filed on Mar. 31, 2003, now issued as U.S. Pat. No.6,663,225, which is a continuation of U.S. application Ser. No.09/864,332 filed on May 25, 2001, now issued as U.S. Pat. No. 6,540,331,which is a continuation of U.S. application Ser. No. 09/112,767 filed onJul. 10, 1998, now issued as U.S. Pat. No. 6,416,167, the entirecontents of which are herein incorporated by reference.

U.S. application Ser. Nos. 09/864,332, 09/112,767 and 09/112,768 arehereby incorporated by reference.

FIELD OF THE INVENTION

This invention relates to a micro-electromechanical device having alaminated thermal bend actuator.

BACKGROUND OF THE INVENTION

The applicant has invented a page width printhead which is capable ofgenerating text and images of a resolution as high as 1600 dpi.

The printheads are manufactured in accordance with a technique that isbased on integrated circuit fabrication. An example of such a techniqueis that which is presently used for the fabrication ofmicro-electromechanical systems.

These fabrication techniques allow the printhead to incorporate up to84000 nozzle arrangements. The nozzle arrangements areelectromechanically operated to achieve the ejection of ink.

In a number of the Applicant's inventions, the nozzle arrangementsincorporate thermally actuated devices which are displaceable withinnozzle chambers to eject the ink from the nozzle chambers. Many of thethermal actuators use a combination of materials and a bending actionwhich results from an uneven expansion of the materials. The thermalactuators are manufactured by depositing consecutive layers of materialhaving different coefficients of thermal expansion.

The present invention was conceived to address certain problemsassociated with such actuators. A significant problem with suchactuators is that the different materials can result in bending andbending stresses being set up in the thermal actuator when the thermalactuator is inoperative and exposed to transient conditions. As is knownin the field of integrated circuit fabrication, the deposition ofmaterial results in a heating of both the material being deposited andthe material on which the deposition takes place. The fact that thematerials have different thermal expansion characteristics can result inthe bending of the laminated structure upon cooling. This is also thecase where the materials have different elasticity characteristics.Those skilled in the field of micro electromechanical systemsfabrication will appreciate that this is highly undesirable.

SUMMARY OF THE INVENTION

According to a first aspect of the invention, there is provided amicro-electromechanical device which comprises

a substrate containing drive circuitry; and

an elongate actuator that is fast with the substrate at a fixed end, theelongate actuator having a laminated structure of at least one innerlayer and a pair of opposed, outer layers, the outer layers havingsubstantially the same thermal expansion and elasticity characteristics,with one of the outer layers defining an electrical heating circuit thatis in electrical contact with the drive circuitry to be heated and toexpand on receipt of an electrical signal from the drive circuitry andto cool and contract on termination of the signal, thereby to generatereciprocal movement of the actuator.

The actuator may have a single inner layer.

The outer layers may have a higher coefficient of thermal expansion thanthe inner layer.

According to a second aspect of the invention, there is provided amicro-electromechanical device which comprises

a substrate containing drive circuitry; and

a plurality of elongate actuators, each actuator being fast with thesubstrate at a fixed end, each elongate actuator having a laminatedstructure of at least three layers in the form of a pair of opposed,outer layers and at least one inner layer, the outer layers havingsubstantially the same thermal expansion and elasticity characteristics,with one of the outer layers defining an electrical heating circuit thatis in electrical contact with the drive circuitry to be heated and toexpand on receipt of an electrical signal from the drive circuitry andto cool and contract on termination of the signal, thereby to generatereciprocal movement of the actuator.

According to a third aspect of the invention, there is provided a fluidejecting device which comprises

a substrate containing drive circuitry,

nozzle chamber walls and a roof wall positioned on the substrate todefine a nozzle chamber in which fluid is received and a fluid ejectionport from which the fluid is ejected, in use;

a fluid ejecting mechanism that is operatively arranged with respect tothe nozzle chamber to act on the fluid in the nozzle chamber to ejectfluid from the fluid ejection port;

a thermal bend actuator that is connected to the drive circuitry toreceive an electrical signal from the drive circuitry and to provideactuation of the fluid ejecting mechanism, wherein the thermal bendactuator has a laminated structure of at least three layers in the formof a pair of opposed, outer layers and at least one inner layer, theouter layers having substantially the same thermal expansion andelasticity characteristics.

The thermal bend actuator may have a single inner layer.

The outer layers of the thermal bend actuator may each be conductive.

At least one of the outer layers of the thermal bend actuator may beconnected to the drive circuitry so that said at least one of the outerlayers can be heated.

The outer layers may have a higher coefficient of thermal expansion thanthe inner layer.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 shows two conditions of a thermal bend actuator of a fluidejection device, not in accordance with the invention, and indicatingthe problem associated with such thermal bend actuators;

FIG. 2 shows a schematic view of a thermal bend actuator of a fluidejection device, in accordance with the invention, and, in particular,the advantage associated with such a thermal bend actuator; and

FIG. 3 shows a fluid ejection device in accordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1, reference numeral 10 generally indicates an actuatingmechanism in the form of a bi-layer thermal bend actuator.

As set out above, the device in which the thermal bend actuator 10 is tobe incorporated is formed as part of an integrated circuit fabricationprocess. It follows that the thermal actuator 10 is manufactured in adeposition and etching process. Thus, once a first layer 12 has beendeposited and prepared, a second layer 14 is deposited on the firstlayer 12. In order to operate correctly, one of the layers, in this casethe first layer 14 is of a material having a higher coefficient ofthermal expansion than the material of the second layer 12.

As is well known in the field of integrated circuit fabrication,deposition of material occurs at a temperature which is, of necessity,significantly higher than ambient temperature. This results in a heatingof the first layer 12 and the deposited second layer 14.

Also, in order to operate, the layers 12, 14 are of materials which havedifferent coefficients of thermal expansion. It follows that, uponcooling after deposition, thermal stresses are set up between the layers12, 14 which can cause bending of the actuator 10. This is extremelyundesirable, particularly in light of the fact that the actuators aremanufactured on a micro-electromechanical scale.

In FIG. 2, reference numeral 20 generally indicates an actuatormechanism of a fluid ejection device, in accordance with the invention.

The actuator mechanism 20 includes a thermal bend actuator 22 which hasthree layers in the form of a pair of opposed outer layers 24 and aninner layer 26.

The outer layers 24 are of substantially the same material and are ofsubstantially the same dimensions. Further, the outer layers 24 are eachconductive.

The outer layers 24 are of a material having a coefficient of thermalexpansion which is such that, upon heating of any one of the layers 24,the actuator 22 bends to a degree sufficient to perform work. Inparticular, the outer layers 24 can be of any material having a suitableYoung's modulus and coefficient of thermal expansion. Possible materialsare titanium nitride and a copper nickel alloy.

The inner layer 26 can be any suitable insulating material such as glass(amorphous silicon dioxide) or even air.

It will be appreciated that the thermal bend actuator 22 will findapplication in any micro electromechanical system in which a prime moveris required. Thus, at least one of the outer layers 24 is connectable todrive circuitry of such a micro electromechanical device.

In FIG. 3, reference numeral 30 generally indicates a fluid ejectiondevice in accordance with the invention. In this embodiment, the fluidejection device is in the form of a nozzle arrangement of an ink jetprinthead, which includes the actuating mechanism 20.

It is to be appreciated that reference to the nozzle arrangement 30 isfor illustrative purposes and should not be construed as limiting theinvention to this particular embodiment.

The nozzle arrangement 30 is formed on a wafer substrate 32 in asuccessive deposition and etching process which forms part of anintegrated circuit fabrication technique conventionally used in themanufacture of micro electromechanical systems.

In this particular example, the nozzle arrangement 30 is formed on adrive circuitry layer 34 which, itself, is formed on the wafer substrate32.

A support post 36 extends from the drive circuitry layer 34. The thermalbend actuator 22 is mounted, cantilever-fashion, on the support post 36.One of the outer layers 24 is in electrical contact with the drivecircuitry layer 34 so that movement of the bend actuator 22 can beachieved with a control system (not shown) connected to the drivecircuitry layer 34.

A cylindrical wall 38 is formed on the drive circuitry layer 34 todefine a nozzle chamber 40. A roof wall 42 is arranged on thecylindrical wall 38 and defines an ink ejection port 44 from which inkis ejected out of the nozzle chamber 40. An ink ejection member 46 ismounted on the thermal bend actuator 22 and extends through a slot 48defined in the cylindrical wall 38. The ink ejection member 46 includesan arm 50 and a paddle 52 mounted on the arm 50 and being shaped tocorrespond generally with a cross-sectional dimension of the nozzlechamber 40.

The slot 48 in the cylindrical wall 38 is shaped to define a guideformation 54 in the cylindrical wall 38. An end of the arm 50 on whichthe paddle 52 is mounted is shaped to correspond with the guideformation 54. In particular, the guide formation 54 and the end 56 ofthe arm 50 are shaped so that, on bending of the bend actuator 22,movement of the end 56 and hence the paddle 52 is retained along alinear path.

The nozzle arrangement 30 is one of a plurality of nozzle arrangementsformed on the wafer substrate 32 to define the ink jet printhead of theinvention. It is simply for reasons of clarity and ease of descriptionthat a single nozzle arrangement is shown in the accompanying drawings.

It will be appreciated that, due to the fact that each nozzlearrangement is a micro-electromechanical device and that up to 84000such nozzle arrangements may be required for a single printhead,accuracy and consistency of manufacture of each nozzle arrangement isextremely important. It would therefore be highly disadvantageous if,upon cooling after deposition, the thermal bend actuator 22 became bentor warped. This would result in an uneven positioning of the paddles 52within the nozzle chambers 40.

Applicant submits that the fact that the two opposed outer layers 24have the same thermal expansion and elasticity characteristics resultsin stability of the bend actuator 22 upon cooling after deposition. Inthis manner, consistently straight bend actuators 22 can be achieved.

A further advantage that has been identified by the Applicant is that,in general operation, the substantially identical outer layers 24 of thethermal actuator 22 provide a high level of thermal stability. Thisallows the thermal actuator 22 to be operated repeatedly in spite of thefact that all the heat from previous activations has not yet dissipated.

1. A micro-electromechanical device which comprises a substratecontaining drive circuitry; and an elongate actuator that is fast withthe substrate at a fixed end, the elongate actuator having a laminatedstructure of at least one inner layer and a pair of opposed, outerlayers, the outer layers having substantially the same thermal expansionand elasticity characteristics, with one of the outer layers defining anelectrical heating circuit that is in electrical contact with the drivecircuitry to be heated and to expand on receipt of an electrical signalfrom the drive circuitry and to cool and contract on termination of thesignal, thereby to generate reciprocal movement of the actuator.
 2. Adevice as claimed in claim 1, in which the actuator has a single innerlayer.
 3. A device as claimed in claim 1, in which the outer layers havea higher coefficient of thermal expansion than the inner layer.
 4. Amicro-electromechanical device which comprises a substrate containingdrive circuitry; and a plurality of elongate actuators, each actuatorbeing fast with the substrate at a fixed end, each elongate actuatorhaving a laminated structure of at least three layers in the form of apair of opposed, outer layers and at least one inner layer, the outerlayers having substantially the same thermal expansion and elasticitycharacteristics, with one of the outer layers defining an electricalheating circuit that is in electrical contact with the drive circuitryto be heated and to expand on receipt of an electrical signal from thedrive circuitry and to cool and contract on termination of the signal,thereby to generate reciprocal movement of the actuator.